1. Field of the Invention
This invention relates to a picture data recording apparatus for recording picture data onto a recording medium such as a magnetic tape or an optical disk, etc. and a picture data reproducing apparatus for reproducing picture data recorded on the recording medium, and more particularly to a picture data recording apparatus and a picture data reproducing apparatus for implementing efficient coding processing to picture data to carry out recording/reproduction of the coded data.
2. Description of the Prior Art
When transmitting picture data, or recording it onto a recording medium, e.g., a magnetic tape, etc., various coding methods for compression of picture information are employed. For example, predictive coding, transform coding, and vector quantization, etc. are known.
Meanwhile, the transform coding utilizes correlation that picture signals have to transform sampled values (hereinafter referred to as picture data) to data on axes perpendicular to each other to allow the correlation between picture data to be lost to carry out reduction of data quantity. As this transform coding system, there is employed an orthogonal transform system in which so called basic vectors are perpendicular to each other, the sum total of mean signal powers which have not undergone transform processing and the sum total of mean powers of so called transform coefficients obtained by orthogonal transform processing are equal to each other, and the power concentration on the low frequency component is excellent. Regarding the orthogonal transform coding, there are known, for example Hadamard Transform, Haar Transform, Karhunen-Loeve (K-L) Transform, Discrete Cosine Transform (hereinafter referred to as DCT), Discrete Sine Transform (hereinafter referred to as DST), and Slant Transform, etc.
Here, the above-mentioned DCT will be briefly described.
In the DCT, a picture (frame) is divided into picture blocks comprised of n pixels disposed in both horizontal and vertical directions, i.e., (n.times.n) pixels at a spatial arrangement to implement orthogonal transform processing to picture data within the picture blocks by using a cosine function. In this DCT, a high speed operational algorithm exists. Because a LSI of one chip permitting an actual time transform processing of picture data has been realized, such DCT is widely used in transmission or recording of picture data. Further, the DCT processing has, as the coding efficiency, a characteristic substantially equal to that of the above-mentioned K-L transform processing which is an optimum transform processing in respect of the power concentration on the low frequency component which directly affects the efficiency. Accordingly, transform coefficients obtained by the DCT are caused to undergo such a processing to encode only components where powers concentrate, thereby permitting a quantity of information to be reduced to a high degree as a whole.
In a more practical sense, when transform coefficients obtained by implementing DCT processing on picture data are represented with, e.g., C.sub.ij (i=0.about.n-1, j=0.about.n-1), the transform coefficient C.sub.00 corresponds to a d.c. component indicating a mean luminance value within a picture block, and its power ordinarily becomes considerably larger than those of other components. In the case where this d.c. component is coarsely quantized, there is high possibility that there may take place a so called block distortion which is a noise peculiar to the orthogonal transform coding that visually deteriorates the picture quality. For this reason, a greater number of bits are allocated to the transform coefficient C.sub.00 to equally carry out quantization. On the other hand, for transform coefficients C.sub.ij of other components, except for the d.c. component (i.e., except for C.sub.00), e.g., by making use of the visual characteristic that the visual spatial frequency lowers in a higher frequency band, weighting is carried out so as to quantize those transform coefficients in such a manner that as the frequency shifts to a higher frequency side, the number of bits allocated thereto becomes lesser.
In transmission or recording of picture data, transform coefficients obtained by implementing DCT processing are quantized as described above, and are then caused to undergo variable length coding such as Huffman coding, or Run Length coding, etc. for the purpose of carrying out data compression to add a synchronizing signal and/or parity code, etc. to the coded data thus obtained thereafter to carry out transmission or recording thereof.
Meanwhile, in a video tape recorder adapted for recording a video signal onto a magnetic tape as a digital signal (hereinafter simply referred to as a VTR), it is required to allow a quantity of data of one frame or one field to be fixed (have a fixed length) when editing and/or variable speed reproduction, etc. are taken into consideration. Since a quantity of data generated can be controlled by varying the quantization step width, an approach is employed to vary the quantization step width while calculating a quantity of data generated so that it becomes equal to a predetermined target data quantity.
However, a quantity of data generated diversely changes depending upon a video signal to be handled, so there is no guarantee that all picture data have a data quantity which falls within a predetermined target data quantity, Accordingly, although such data exceeding a predetermined target data quantity are generated at a very low probability in an actual sense, it is necessary to ensure that the recording capacity does not exceed the maximum data quantities of all picture data generated. For this reason, the memory capacity is required to have a remarkable margin. As a result, realization of high density recording of the VTR was hindered.